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A molecular dynamics study on transport properties and structure at the liquid-vapor interfaces of alkanes

机译：烷烃在液-气界面的输运性质和结构的分子动力学研究

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In this paper, self-diffusion and structure in the vicinity of liquid-vapor interfaces of several species of n-alkanes are examined using molecular dynamics simulations. The united atom NERD force field was utilized to model the alkane molecules, and the obtained properties were validated by the reported experimental results. Surface tension was calculated based on the averaged pressures, and tail corrections were implemented based on the molecular virial formulae. Present surface tension results for butane, hexane and decane are in good agreement with experimental and other simulated values. Molecular orientation and chain conformation in the interface region were examined by observing the ordering parameter and radius of gyration for decane and tetracosane. In the interface region, molecules are preferentially oriented parallel to the interface and shape of the molecules is slightly flattened in the z-direction for both decane and tetracosane. With an increase in temperature, molecules are preferentially less parallel to the interface. The Einstein relation modified for confined regions was used to obtain the self-diffusion coefficient according to migration of molecules parallel to the interface. Self-diffusion coefficient increases in the interface region for both decane and tetracosane. This increase in self-diffusion coefficient has a correlation with decrease in density. The dependency of self-diffusion coefficient on density is nonlinear in the liquid and vapor side of the interface region and is linear in between the above two regions. Influence of chain length of the alkane molecules has been studied by comparing the physical and structural quantities of decane, tetracosane and hexatriacontane at the same reduced temperature. At the same reduced temperature, self-diffusion coefficient decreases with an increase in chain length in the interface region as well as in the bulk liquid region. All three alkanes show similar tendency of ordering and chain conformation at the interface at the same reduced temperature.
机译：在本文中，使用分子动力学模拟研究了几种正构烷烃的液-气界面附近的自扩散和结构。利用联合原子NERD力场对烷烃分子进行建模，并通过报道的实验结果验证了所获得的性质。根据平均压力计算表面张力，并根据分子病毒式进行尾部校正。目前丁烷，己烷和癸烷的表面张力结果与实验值和其他模拟值非常吻合。通过观察癸烷和十四烷的排序参数和回转半径来检查界面区域的分子取向和链构象。在界面区域中，分子优选平行于界面取向，并且对于癸烷和十四烷均分子的形状在z方向上稍微变平。随着温度的升高，分子优先较少地平行于界面。根据有限的区域修改的爱因斯坦关系用于根据平行于界面的分子的迁移获得自扩散系数。癸烷和十四烷均在界面区域自扩散系数增加。自扩散系数的这种增加与密度的降低相关。自扩散系数对密度的依赖性在界面区域的液体和蒸气侧是非线性的，而在上述两个区域之间是线性的。通过比较在相同的还原温度下癸烷，十四烷和六十六烷的物理和结构量，研究了烷烃分子链长的影响。在相同的降低温度下，自扩散系数随着界面区域以及本体液体区域中链长的增加而降低。在相同的降低温度下，所有三种烷烃在界面处均显示出相似的有序化和链构象化趋势。

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